In recent years, an aqueous ink that is safe and has small environmental impact has been used in various fields instead of an organic solvent ink. In business applications, an odorless aqueous coloring material is indispensable as an ink used for printing in offices. In industrial applications, there has been an increasing tendency to reduce the use of an organic solvent as much as possible from the viewpoint of an improvement in working environment, an improvement in safety of ink and paint, and waste disposal. An aqueous coloring material also has an advantage in that the production cost can be reduced as compared with an organic solvent coloring material since a special explosion-proof system and a ventilator are unnecessary during production.
A dye or a pigment has been mainly used as the aqueous coloring material depending on the application. A dye can easily form a high-resolution image that exhibits an excellent tone, but exhibits poor light resistance as compared with a pigment. A pigment exhibits poor dispersibility as compared with a dye, but exhibits excellent water resistance and light resistance. A number of pigment inks have been developed along with the development of dispersion technique.
An aqueous ink for gravure printing or offset printing has been developed. An inkjet recording method has been most widely used as an aqueous ink recording method for office applications. The inkjet recording method discharges ink droplets from a minute nozzle head to record characters or figures on the surface of a recording medium (e.g., paper). The noncontact inkjet recording method can implement easy on-demand full-color printing of an image on various recording media (e.g., plain paper) without requiring a printing plate.
An inkjet printer that utilizes the inkjet recording method discharges ink droplets from a minute nozzle head to record characters or figures on the surface of a recording medium (e.g., paper). Typical inkjet printing methods include a Bubble Jet (registered trademark) method and a piezo method. The Bubble Jet method instantaneously heats an ink introduced into the nozzle head using a heater to produce bubbles, and intermittently discharges the ink by utilizing the volume expansion due to the bubbles. The piezo method converts an electrical signal into a mechanical signal using an electrostrictive element (piezoelectric element), and intermittently discharges ink droplets stored in the nozzle head.
A black inkjet printer ink composition has been widely used in offices and the like mainly in order to print documents. A number of aqueous black pigment ink compositions that are optimized for the recording method and the recording speed of each printer have been proposed.
For example, Patent Document 1 discloses an aqueous black pigment ink composition that is provided with improved dispersibility in an aqueous medium by hydrophilizing the surface of the black pigment. However, since the ink composition disclosed in Patent Document 1 may penetrate when printed on plain paper, a decrease in image density, offset, curling of the recording paper, and the like may occur. In particular, since the amount of ink discharged per unit time has increased along with an increase in printing speed of inkjet printers, a sufficient image density may not be obtained when using a related-art aqueous black pigment.
Since a pigment exhibits a strong interparticle cohesive force, but exhibits weak affinity to an organic polymer, water, an organic solvent, and the like, it is very difficult to uniformly mix or disperse a pigment under normal mixing or dispersion conditions. In order to deal with this problem, attempts have been made to improve the dispersibility of a pigment by improving the affinity of the pigment to a solid or liquid component by causing a dispersant (e.g., surfactant or resin) to be adsorbed on the surface of the pigment, or coating the entire surface of the pigment with the dispersant (see Patent Document 2, for example).